1. Field of the Invention
This invention relates to a thermoplastic block copolymer, to a method for preparing said block copolymer and to compositions comprising said block copolymer. More particularly, this invention relates to a thermoplastic block copolymer comprising a polyester block, to a method for preparing said thermoplastic block copolymer and to thermosetting compositions comprising said block copolymer.
2. Prior Art
The use of unsaturated polyesters in thermosetting resin compostions, is, of course, well known in the prior art. Generally, these compositions comprise an unsaturated polyester, a vinyl monomer and a curing agent. The thermosetting resin compositions may also comprise other additives such as fillers, reinforcing agents, antishrinking agents, thickeners and the like.
As is also well known in the prior art, the unsaturated polyester resin compositions, generally, exhibit excellent rigidity, heat resistance and electrical characteristics when used in thermosetting applications. Products, particularly molded products, prepared with unsaturated polyester resin compositions do not, however, generally exhibit good impact resistance or good surface characteristics. The poor surface characteristics are, generally, attributed to shrinkage during curing of the composition. As a result, use of the unsaturated polyester resin compositions in certain applications such as bulk molding and sheet molding applications has been restricted.
In recent years, considerable effort has been made to improve both the impact resistance and surface characteristics of the unsaturated polyester resin composition products so as to expand the areas in which unsaturated polyester resin compositions may be used. Particular emphasis has been placed on using these compositions in bulk molding and sheet molding applications. For example, and as indicated in the background section of U.S. Pat. No. 4,329,438 several methods have, heretofore, been proposed for improving the impact resistance and the surface characteristics of such unsaturated polyester resin compositions. These methods include the addition of either a diolefin rubber or a styrene-diolefin block copolymer to the resin compositions. Further, the use of carboxylated derivatives of styrene-butadiene block copolymers in polyester resin compositions has been tried. As indicated in the background section of said U.S. Pat. No. 4,329,438, however, these methods have not been completely successful due, primarily, to the incompatability of the rubber polymer in the composition and the resulting destructive phase separation which leads to bleeding of the rubber to the surface during aging of the molding paste. Another difficulty with the proposed prior art solutions to the problems, particularly in those cases where a block copolymer is used, is solubility of the polymer in the vinyl monomer which functions both as a crosslinking agent and as a solvent in the composition.
U.S. Pat. No. 4,329,438 teaches yet another method for solving the destructive phase separation problem wherein a carboxylated derivative of a styrene-butadiene block copolymer is used. Specifically, the styrene-butadiene block copolymer is modified by grafting an unsaturated dicarboxylic acid and/or an unsaturated dicarboxylic acid derivative to the block copolymer. According to the disclosure, significant improvement is realized but destructive phase separation and the associated rubber bleeding is apparently not completely eliminated since the block copolymer and the unsaturated polyester apparently remain as a physical admixture in the thermosetting resin composition, at least during any storage or aging thereof. Moreover, since unsaturated block copolymers are used, structures fabricated with the thermosetting resin compositions exhibit poor thermal and oxidation stability.
U.S. Pat. No. 4,020,036 teaches a method for improving impact resistance and surface characteristics similar to that taught in U.S. Pat. No. 4,329,438. It is, however, within the scope of the teaching of U.S. Pat. No. 4,020,036 to prepare and use a carboxyl terminated conjugated diene polymer. The use of a conjugated diolefin polymer having a terminal carboxyl group in the composition of U.S. Pat. No. 4,020,036 would not, however, result in the production of a thermoplastic block copolymer having a polyester block as is contemplated in the present invention. In fact, the composition taught in U.S. Pat. No. 4,020,036 remains a physical admixture subject to destruction phase separation, at least until the composition is cured. After curing, the composition is "thermoset", not "thermoplastic". Moreover, the conjugated diolefin polymer contemplated for use in the compositions taught in U.S. Pat. No. 4,020,036 are unsaturated. Products prepared therewith, therefore, remain subject to oxidative and thermal degradation.
The use of a hydrogenated conjugated diolefin block to improve weatherability and resistance to thermal deterioration is either taught or at least inherent in the polymer compositions taught in U.S. Pat. Nos. 4,400,478 and 4,657,970. Both of these patents also teach the incorporation of carboxylic acid groups into the polymeric modifier for the purpose of improving impact resistance and surface characteristics. The carboxylic acid groups are, however, incorporated via grafting and, as a result, the number of such acid groups actually incorporated is at best difficult to control. To the extent that these carboxyl groups are, then, reacted with a terminal hydroxyl group contained in the ethylenically unsaturated polyester resin the number of polyester resin segments actually incorporated into the polymer would be difficult to control. Moreover, since the carboxyl groups are, in effect, randomly distributed along the elastomer portion of the polymer, the resulting polymer would not, strictly, be classed as a block copolymer.
In light of the foregoing, it is believed apparent that considerable effort has been expended in the development of an unsaturated polyester resin composition that would be useful in bulk molding and sheet molding applications. As also believed apparent from the foregoing, this effort has not yet produced such a composition exhibiting good improvement in such properties as impact resistance, weatherability and resistance to thermal deterioriation and at the same time permitted good control over the ratio of unsaturated polyester segments to modifier segments in the thermosetting resin composition. The need, therefore, remains for such an improved unsaturated polyester resin composition.